Displays employing various illumination sources are known in the art. Among these are displays employing a light modulator, such as liquid crystal displays, which are illuminated by a separate light source, typically a fluorescent tube or inorganic light-emitting diode. While displays employing this technology are dominant in today's market they often are much less energy efficient than desired since light is typically emitted in areas of the display even when the light modulator is closed, precluding the light from being observed by the user of the display. Displays which directly modulate the emission of light, such as displays employing Organic Light Emitting Diodes (OLEDs) have the potential to produce light much more efficiently and with higher visual quality. Therefore, OLED displays have the potential to displace LCD displays, especially in markets in which power consumption is a key attribute of the display. These OLED devices can additionally be employed in other devices that require the color of emitted light to be adjusted, including lamps with adjustable color.
Unfortunately, OLED technology, particularly OLED display technology, has been adopted slowly. This slow adoption rate stems primarily from the high cost of producing such displays. In this technology one of the largest challenges is the formation of full-color displays by forming an array of light-emitting elements capable of emitting multiple colors of light. Various approaches to full color have been attempted. Patterning of different colors of material by vapor deposition of organic materials through shadowmasks has proven to be effective. However these shadowmasks limit the resolution of the displays, the size of the substrate that can be successfully coated, and the tact time. Other approaches, such as the use of laser deposition to pattern color emitters has been demonstrated but this technology often produces displays with low yields and often results in significant residual waste. Solution printing of different colored organic emitters has also been discussed but these processes typically result in emitters with significantly lower emission efficiency as compared to emitters deposited by vapor deposition through a shadowmask. This lower efficiency is due to the fact that polymeric materials often have a lower luminescent efficiency and lifetime than small molecule materials and because only a small number of layers can be solution deposited on one another to manage the movement of carriers through the organic layer. Other approaches to forming multicolor OLED devices have also been attempted, including the use of a white emitter together with patterned color filters. However, these approaches also reduce the effective efficiency of the emitters within the OLED display.
In inorganic electronic devices, it is known to apply photolithographic techniques to pattern multiple thin film layers of inorganic semiconductors and inorganic electrically conductive layers with high resolution over large substrates for forming arrays of electrical components. Unfortunately, the photolithographic materials and solvents applied to form these devices are known to dissolve organic materials. Therefore, it is not possible to apply the photolithographic materials and solvents that are known to be used to manufacture inorganic solid state circuits to pattern layers of organic material, especially layers that include active semiconductive organic materials or layers that are formed on top of organic materials.
Recently photoresist materials and solvents have been discussed in the art to facilitate the use of photolithographic techniques to pattern polymeric organic semiconducting layers. For example, Zakhidov et al. in an article published in Advanced Materials in 2008 on pages 3481-3484 and entitled “Hydrofluoroethers as Orthogonal Solvents for the Chemical Processing of Organic Electronic Materials” discusses a method for patterning polymer organic material in which a fluorinated photoresist is deposited on a substrate, selectively exposed to an energy source to render insoluble through deprotection a portion of the photoresist, the photoresist is developed in a solvent including hydrofluoroether to develop the pattern and remove the portion of the photoresist material that was not deprotected. The solubility of the deprotected photoresist in a hydrofluoroether was then reestablished through the use of another solvent. An active organic semiconductor was then deposited over the remaining photoresist and remaining photoresist was lifted off to pattern the active organic semiconductor. As such, this paper demonstrates the patterning of a single solution-coated, polymeric, organic semiconductor on a substrate. The same general process has been discussed by Lee et al. in an article published in the Journal of the American Chemical Society in 2008 on pages 11564 through 11565 and entitled “Acid-Sensitive Semiperfluoroalkyl Resorcinarene: An Imaging Material for Organic Electronics”.
Taylor et al. in an article published in Advanced Materials on Mar. 19, 2009 on pages 2314-2317 and entitled “Orthogonal Patterning of PEDOT:PSS for Organic Electronics using Hydrofluoroether Solvents” discusses the formation of a bottom contact thin film transistor in which a polymeric organic conductor (i.e., PEDOT:PSS) is formed on a substrate, a photoresist is formed and patterned over the conductor, the conductor is etched, a second photoresist is applied and patterned before an organic semiconductor (i.e., Pentacene) is applied and patterned.
While each of these papers discuss patterning of solution-coated, polymeric organic materials using a modified photolithographic process and materials to create components in an electrical circuit, the use of processes and materials such as these have not been applied to OLED devices. Further, these papers discuss the application of these materials and processes for use with polymers and do not provide a method for patterning layers of small molecule organic materials. There is therefore a need for a method to produce a multicolor or full-color OLED device on a large substrate. It is especially desirable that this method be applicable to forming highly efficient devices which employ vapor deposited, small molecule organic materials.